As a method for treating aneurysm or the like, which causes little invasion, there is currently known vascular embolization in which an intracorporeally indwelling member composed of a metal, which does not exert an adverse influence on a vital body, such as platinum is left within aneurysm. In this method, a medical wire with the intracorporeally indwelling member connected to the leading end part of a delivery wire is inserted into a catheter arranged in the vital body, the wire is operated under observation by a radiography to guide the intracorporeally indwelling member to a desired position within the vital body, whereby the intracorporeally indwelling member is caused to reach the intended site, and the intracorporeally indwelling member is released in this state.
As a method for releasing the intracorporeally indwelling member, there are known mechanical means and electrical means. For example, as an electrically releasing means, there is a means of supplying an electric current between a conductive wire and a counter electrode connected to the vital body from the outside to decompose and fuse a connecting member (see, for example, Patent Art. 1 and Patent Art. 2).
Patent Art. 2 discloses a medical wire for vascular embolization in which an intracorporeally indwelling member is connected to the leading end part of a conductive wire through a thermally-fusible rod-like connecting member composed of polyvinyl alcohol. According to this medical wire, a high-frequency electric current is applied between the conductive wire and the counter electrode, whereby the leading end part of the conductive wire functions as an electrode for heating to thermally fuse the connecting member in a moment, and the intracorporeally indwelling member is separated from the conductive wire, so that the medical wire is said to have merits that the time required for a surgical operation can be shortened and burdens imposed on a patient and a surgeon can be lightened.    Patent Art, 1: Japanese Patent Registration No. 3007022; and    Patent Art. 2: Japanese Patent Registration No. 2880070.
However, the rod-like connecting member composed of polyvinyl alcohol changes its form in the course of a swelling treatment with water or the like, which is required to use the medical wire, in such a manner that it expands in its radial direction and contracts in its axial direction (lengthwise direction), and moreover water is absorbed in the interior thereof by swelling, whereby it becomes flexible compared with that in a dry state, and so it is easily curved or bent. As a result, the connecting member composed of polyvinyl alcohol is crushed or bent in the catheter upon delivery of the intracorporeally indwelling member by the operation of the conductive wire when a route to an affected part is complicated, and a delivery resistance is high, so that the intracorporeally indwelling member may come into contact with the electrode part of the leading end of the conductive wire in some cases.
When it is specifically described, FIG. 5 is a cross-sectional view illustrating, on an enlarged scale, a joint portion between a conductive wire and an intracorporeally indwelling member in an exemplary conventional medical wire. As shown in FIG. 5, a coiled electrode part 12 is formed at a leading end part of a conductive wire 10 in this medical wire, a base end portion 14A of a rod-like connecting member 14 composed of polyvinyl alcohol is inserted into the coil of this electrode part 12 and fixed with an adhesive 16, a leading end portion 14B of the connecting member 14 is inserted into a coil forming an intracorporeally indwelling member 18 and fixed with an adhesive 20 likewise, the leading end of the electrode part 12 of the conductive wire 10 and the base end of the intracorporeally indwelling member 18 are separated from each other with a slight space, and the connecting member 14 is in an exposed state at this space portion. Reference numeral 22 designates a fluorocarbon resin coating provided on other portion than the leading end part and base end part of-the conductive wire 10 and forming an insulating surface.
When the medical wire of such construction is subjected to a swelling treatment for use by placing it in water, however, polyvinyl alcohol forming the connecting member 14 absorbs water and swells. As a result, a portion of the connecting member 14, which is not fixed by the adhesives 16 and 20, expands and swells out in its radial direction and contracts in its axial direction as illustrated in FIG. 6, so that the leading end of the electrode part 12 of the conductive wire 10 approaches the base end of the intracorporeally indwelling member 18, and both may become a contacted state in some cases. Reference character 14S indicates a swollen-out portion formed in the connecting member 14.
In the state that the electrode part 12 of the conductive wire 10 has come into contact with the intracorporeally indwelling member 18, a high-frequency current flows into the intracorporeally indwelling member 18 when the current is supplied to the conductive wire 10, so that a member acting as the electrode part becomes a state that its surface area has increased to create a state that an impedance of a circuit formed by a power source device, the conductive wire 10, a vital body and a counter electrode has greatly decreased, and so the electrode part 12 does not sufficiently generate heat. As a result, fusion of the connecting member 14 is not surely achieved. After all, a situation that the intracorporeally indwelling member 18 cannot be released is brought about. This is attributable to the fact that heat (Joule heat) generated in the electrode part 12 depends on the impedance of the circuit, and the quantity of heat generated decreases when the impedance is low.